This invention relates to a method of oxidizing a nitrous acid radical (NO.sub.2.sup.-) contained in an aqueous solution, particularly plant waste water with hydrogen peroxide to convert said nitrous acid radical into a nitric acid radical (NO.sub.3.sup.-).
The method of this invention can be effectively applied in treating plant waste water chiefly containing a nitrous acid radical. The reason for this is that a nitrous acid radical contained in a plant waste water acts to increase the value of chemical oxygen demand (hereinafter referred to as "COD") of said waste water, and the nitrous acid has to be properly treated. As used herein, the term "COD value" is defined to mean an amount of oxygen (expressed in ppm) equivalent to that of potassium permangate required to oxidize plant waste water.
The nitrous acid radical is contained in waste water discharged from the plants of organic nitration and nitric acid oxidation and nitrogen oxide-absorbing plants. A high temperature heating medium composed of a molten salt mixture of nitrate and nitrite is widely applied in the fields of, for example, the heat treatment of metals, the processing of soft polyvinyl chloride and the vulcanization of rubber products. Where, for example, a metal material is treated in direct contact with the molten salt mixture, then it is necessary to wash the treated material with clear water. In this case, however, a small amount of nitrite is unavoidably carried into the waste water, after washing.
The various methods available to treat a nitrous acid radical contained in the waste water discharged from the above-mentioned plants are essentially based on the oxidization of said nitrous acid radical into a nitric acid radical which does not increase the COD value of the waste water. One of the proposed methods is to carry out said oxidation by oxides of heavy metals. However, this method is liable to give rise to another environmental pollution. Another proposed method is based on photo-oxidation. However, this method has too low an efficiency to be put to practical application. The present inventors have made studies over a broad range to develop a clean method of oxidizing the nitrous acid radical which can be effectively applied on an industrial basis and is not accompanied with any environmental pollution problem after said oxidation. As a result, the inventors have found that where the nitrous acid radical is oxidized by hydrogen peroxide under a certain specified condition, the reaction of said oxidation is fully completed. For application of this finding to the waste water treatment, the inventors has developed an industrial method of automatically oxidizing the nitrous acid radical contained in said waste water into the nitric acid radical which does not increase the COD value thereof.